Maintenance of a nearly constant level of the extracellular calcium concentration (Ca 2+O) is a crucial function of the Ca2+ homeostatic system in humans, because Ca2+ plays numerous crucial biological roles. The extracellular calcium-sensing receptor (CaR) mediates direct actions of Ca2+O on the functions of parathyroid, kidney and other cells participating in Ca2+ homeostasis. The studies in this proposal extend preliminary evidence that the CaR also contributes importantly to the pathophysiology of PTHrP-mediated, humoral hypercalcemia of malignancy as well as to the osteolysis that can be caused by bony metastases of common epithelial cancers (e.g., breast and prostate cancers). This evidence suggests that the CaR causes a homeostatically inappropriate stimulation of parathyroid hormone- related protein (PTHrP) secretion from tumors causing hypercalcemia or metastasizing to bone, thereby producing a vicious cycle in which PTHrP-induced hypercalcemia begets further PTHrP secretion and, in turn, worsening hypercalcemia and/or osteolysis. The overall goal of this proposal is to document and characterize further these potentially key roles of the CaR in the pathophysiology of malignant hypercalcemia and osteolysis. The specific aims of the proposal are to: (1) show that the CaR mediates the stimulatory effects of elevated levels of Ca2+O on PTHrP secretion from the transplantable Rice H-500 leydig cell cancer cell model of humoral hypercalcemia of malignancy and modulates cellular proliferation and/or apoptosis in vitro in these cells; (2) demonstrate that CaR-stimulated PTHrP secretion contributes to the humoral hypercalcemia of malignancy caused in vivo by H-500 cells when transplanted into Fischer rats and that the CaR modulates proliferation and/or apoptosis in vivo in this model; (3) document that the CaR mediates the stimulatory effects of high Ca2+O on PTHrP secretion in vitro from human breast or prostate cancer cell lines observed in preliminary studies and that this receptor modulates the proliferation and/or apoptosis of these cells in vitro; and (4) show that CaR-stimulated PTHrP secretion participates in the osteolysis observed in nude mice injected with breast or prostate cancer cells as a model of malignant osteolysis in humans and that the CaR modulates the proliferation and/or apoptosis of these cells in vivo. These studies could provide novel insights into the pathophysiology of malignancy-associated hypercalcemia and/or osteolysis and provide a foundation for the rational use of CaR-based therapeutics for treating these complications of PTHrP-secreting cancers that metastasize to bone.